This invention relates to methods of collecting projection measurements at a plurality of angles through an object slice, which measurements are used to construct an image of the slice. More specifically, the invention relates to such methods which exhibit reduced sensitivity to motion-related artifacts resulting from object motion during the projection measurement acquisition process. The method has applicability to modalitites utilizing parallel-ray or fan-beam scan geometries. The preferred embodiment will be described with respect to nuclear magnetic resonance (NMR) imaging.
The nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or nuetrons. Due to the spin of the protons and the neutrons, each such nucleus exhibits a magnetic moment, such that when an object composed of such nuclei is placed in a static homogeneous magnetic field, B.sub.o, a greater number of the nuclear magnetic moments align with the field to produce a net macroscopic magnetization, M, in the direction of the field. Under the influence of this field, the magnetic moments precess about the axis of the field. The frequency at which the nuclei precess is dependent on the strength of the applied magnetic field and on the nuclei characteristics. The frequency of precession, .omega., is referred to as the Larmor frequency and is given by the equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio which is constant for each NMR isotope, and B is the strength of the applied magnetic field. This field may include the B.sub.o field as well as magnetic-field gradients which are typically superimposed thereon. It will be recognized, therefore, that the frequency at which the nuclei precess is primarily dependent on the strength of the magnetic field B, and increases with increasing field strength.
It is possible to change the orientation of magnetization M (normally directed along field B.sub.o) relative to the direction of the B.sub.o magnetic field by the application of an oscillating magnetic field which is most advantageously applied by irradiating the object with radio frequency (RF) pulses whose frequency is the same or nearly so as the precession frequency .omega.. Radio-frequency pulses are typically applied in a plane orthogonal to the direction of the B.sub.o field. The resulting magnetic field B.sub.l, resulting from the application of the radio-frequency pulses, causes the magnetization M to precess about the direction of the B.sub.l field farther and farther away from the Z axis (arbitrarily assumed to be the direction of the B.sub.o field). The extent of rotation of magnetization M from the direction of the B.sub.o field is dependent on the intensity and the duration of the RF pulses. A 90.degree. RF pulse, for example, causes magnetization M to depart 90.degree. from the direction of the B.sub.o field into the X-Y plane defined by the X- and Y-axes of the Cartesian coordinate system which in NMR systems is frequently assumed to be rotating at the resonant frequency .omega.. The rotation of the magnetization M into the transverse X-Y plane creates therein a transverse magnetization which is capable of inducing a signal current in an appropriately positioned RF pickup coil, as is well known in the art. The amplitude of the induced signal decreases as the nuclear spins producing the signal dephase or lose their correlation and as the precessing transverse magnetization M returns to its equilibrium state along the B.sub.o field. The observed signal is frequently referred to as the NMR signal, or as the free-induction decay (FID) signal. Another type of RF pulse which is frequently utilized in NMR is a 180.degree. RF pulse which causes magnetization M to rotate by 180.degree. from its original direction (from the positive Z-axis direction to the negative Z-axis dircetion, for example). For this reason, the 180.degree. RF pulse is frequently referred to as the inverting pulse. As will be described hereinafter, 180.degree. RF pulses are frequently utilized to create spin-echo signals. It should be noted that a 90.degree. or a 180.degree. RF pulse will rotate magnetization M through the corresponding number of degrees from any initial direction of magnetization M, provided B.sub.l is perpendicular to M.
It is possible to distinguish NMR signals arising from different spatial position in the sample by changing their respective resonant frequencies. If one or more magnetic-field gradients of sufficient strength to spread out the NMR signal spectrum are applied to the sample, each nuclear spin along the direction of the gradient experiences a different magnetic field strength and, hence, resonates at a different frequency from that of the nuclear spins at other positions along the gradient direction, as predicted by the Larmor equation. Nuclei situated along lines perpendicular to direction of the gradient have the same resonant frequency and their contributions will be superimposed. Thus, the Fourier transform of the measured signal in the presence of a magnetic-field gradient represents a projection in the direction perpendicular to the gradient. In NMR imaging utilizing multiple-angle-projection reconstruction, the gradient direction is varied over a plurality of angles to cover at least a 180.degree. arc in the imaging sample. The NMR signal observed for each gradient direction is Fourier transformed to determine the projections of the object. These projections are then reconstructed into images using well-known techniques, such as the filtered-back-projection technique utilized in X-ray computerized tomography.
In the image-reconstruction process, the projections measured at angles separated by a multiple of 180.degree. in a given scan (either 180.degree. or 360.degree.) mathematically contain identical information about the object. However, the projection angle in the prior art methods is varied monotonically through the angular scan range. Thus, the projections measured at the beginning and end of the scan, in a 180.degree. scan, for example, represent projections along directions that are approximately 180.degree. apart and so should be quite similar. However, since they were measured at opposite ends of the scan time, they may be different if the object moved during the scan. Such differences manifest themselves as streak artifacts in the reconstructed images approximately in the direction of the first (and last) projection measurement. It will be recognized by those skilled in the art that view measurements which are adjacent in view angle within a scan must also be substantially consistent to avoid streak artifacts. It is the principal object of the invention to provide a method in which the sensitivity to such inconsistencies in projection measurement within a scan is reduced thereby to improve image quality.